Device for outputting and/or receiving ultrasound and ultrasound sensor for inspecting a valuable document

ABSTRACT

An apparatus for emitting and/or receiving ultrasound in a predetermined frequency range includes at least one ultrasonic converter for converting ultrasound into electrical signals and/or converting electrical signals into ultrasound, and a holder having at least one ultrasonic channel formed therein in which the ultrasonic converter is disposed at least partly and/or through which ultrasound can pass from or to the ultra-sonic converter. The holder has at least one non-fibrous surface layer adjacent to at least one portion of the ultrasonic channel, whose impedance is smaller than that of an inside layer of the holder neighboring to the surface layer.

The present invention relates to an apparatus for emitting and/orreceiving ultrasound, in particular for examining a value document, andto an ultrasonic sensor for examining a value document.

Value documents are understood here to be sheet-shaped objects thatrepresent for example a monetary value or an authorization and are hencenot to be producible at will by unauthorized persons. Hence, they havefeatures that are not easily produced, in particular copied, whosepresence is an indication of authenticity, i.e. production by anauthorized body. Important examples of such value documents are chipcards, coupons, vouchers, checks and in particular bank notes.

Such value documents are in many cases to be examined by machine fortheir authenticity and/or their physical condition, for example,limpness or the presence of tears, holes or adhesive strips.

For such a check there can be used in known apparatuses for processingvalue documents in particular ultrasound. The corresponding apparatusthen has an ultrasonic sensor which can comprise in particular anapparatus for emitting and/or receiving ultrasound. In suchexaminations, ultrasound can either be emitted continuously or in pulsedfashion onto a value document and the ultrasound thrown back by thevalue document or preferably the ultrasound passed through the valuedocument be detected; then the corresponding detection signals areevaluated.

Such an ultrasonic sensor can also be used in value document processingapparatuses wherein stacks of value documents are singled, fordetermining singling errors, in particular the substantiallysimultaneous removal of two at least partly superposed value documents,also referred to as double or multiple removal.

If the principal direction of propagation or the mean direction ofpropagation of the ultrasound extends at least approximately orthogonalto the surface of the value document, however, there can occur theundesirable effect that the value document throws the ultrasound back inthe direction of an ultrasonic converter used in the ultrasonic sensor.Upon use of continuously emitted ultrasound this, and optionallyreflection on the ultrasonic sensor, can result in standing waves whichfalsify the examination. If the ultrasound is emitted only in the formof pulses, a pulse can be reflected as a so-called pulse echo andlikewise impair the examination.

The present invention is hence based on the object of providing anapparatus for emitting and/or receiving ultrasound, in particular forexamining a value document, or an ultrasonic sensor for examining avalue document, which permit an examination of value documents whichminimizes impairments by undesirable reflections of the employedultrasound.

This object is achieved according to a first alternative by an apparatusfor emitting and/or receiving ultrasound at least of a predeterminedfrequency having at least one ultrasonic converter for convertingultrasound at least of the predetermined frequency into electricalsignals and/or for converting electrical signals into ultrasound atleast of the predetermined frequency, and a holder having formed thereinat least one ultrasonic channel in which the ultrasonic converter isdisposed at least partly and/or through which ultrasound can pass fromor to the ultrasonic converter, the holder having at least onenon-fibrous surface layer adjacent to at least one portion of theultrasonic channel and whose impedance is smaller than that of an insidelayer of the holder neighboring to the surface layer, in particularadjacent thereto.

The inventive apparatus is used for emitting and/or receiving ultrasoundof at least one frequency which is predetermined. Preferably, inparticular for operation with short ultrasonic pulses, the apparatus canalso be configured for ultrasonic conversion in a predeterminedfrequency range which contains the predetermined frequency. For pulsedoperation the width of the frequency range can conform inter alia to thedesired pulse duration. The predetermined frequency or the frequencyrange is preferably within the frequency range of 100 kHz to 1 MHz,particularly preferably between 100 kHz and 600 kHz, for applications inthe area of processing and/or checking value documents. For applicationsin the area of processing and/or checking value documents by means ofultrasonic pulses, the width of the frequency range can be chosen forexample smaller than 70 kHz. The conversion of electrical signals intoultrasound with the predetermined frequency and/or the conversion ofultrasound with the predetermined frequency into electrical signals isdone using the ultrasonic converter, which is configured or chosenaccording to the predetermined frequency or the predetermined frequencyrange. There can be chosen here any, in particular known, ultrasonicconverters, for example converters with a surface circular cylindricalalong the circumference and radiating the ultrasound. The ultrasonicconverter can have on its ultrasound-radiating surface in particular alayer connected to an actuator, for example to a piezoelectric element,specifically for coupling in ultrasound in air.

The ultrasonic converter can be disposed at least partly in theultrasonic channel of the holder, i.e. be countersunk wholly or partlyin the holder. The ultrasonic channel is then so designed that theultrasonic converter can oscillate with the accordingly providedportions freely enough for converting ultrasound. It is also possible,however, that the ultrasonic converter is disposed in a receiving meansin the holder at one end of the ultrasonic channel which opposes the endof the ultrasonic channel through which ultrasound can pass from theholder or into the holder.

The ultrasonic channel can thereby in particular also exert an influenceon the spatial radiating or receiving characteristic of the apparatusfor ultrasound in the predetermined frequency range and be designedaccordingly.

Further, the ultrasonic converter can be disposed with its radiatingdirection in principle at will relative to the holder. Preferably, witha holder having a level surface facing the value document upon anexamination, the ultrasonic converter and preferably the ultrasonicchannel are so aligned that the radiating direction is aligned with anormal to the surface to an angle between 0° and 5°, particularlypreferably between 0° and 1°.

Normally, upon acoustic irradiation of an object by a thus disposedultrasonic converter of a conventional apparatus, at least a portion ofthe ultrasound is thrown back onto the ultrasonic converter, which leadsin continuous operation to standing waves and in pulsed operation tomultiple echoes.

Surprisingly, it has now been found that the formation of standing wavesor the formation of strong multiple echoes can be substantially reducedby the holder having the at least one non-fibrous surface layer adjacentto at least one portion of the ultrasonic channel or at least partlylimiting the latter, whose impedance is smaller than that of an insidelayer of the holder neighboring to the surface layer, in particularadjacent thereto. A non-fibrous surface layer is understood here to be alayer made of a material not present in the form of fibers, inparticular a solid state material including elastic materials or foams.Fibrous surface layers, in contrast, are textile fabrics such asso-called “nonwovens”, for example felts or fleeces, or wovens, knits,etc.

The surface layer can be adjacent to the inside layer in particular inthe radial direction of the ultrasonic channel, or in a directionparallel to the direction of the ultrasonic channel or the direction ofsound propagation in the ultrasonic channel.

This design of the holder has the advantage that non-fibrous materialscan be processed very simply and exactly, while the formation ofportions of an ultrasonic channel in a fibrous material is possible onlyinexactly in particular in the case of small diameters.

The term “impedance” is understood in general within the scope of thepresent invention to be acoustic impedance. In principle it issufficient that the impedance of the surface layer is smaller than thatof the inside layer. Preferably, the impedance of the material of thesurface layer is in the range between 20% and 75% of the impedance ofthe material of the inside layer. In this case an especially goodsuppression of standing waves or multiple reflections is obtained.

The density or impedance of the material of the surface layer isunderstood to be, in the case of inhomogeneous surface layers, theaverage along a normal to the surface of the surface layer through thesurface layer.

The surface layer need in principle not form the entire surface limitingthe ultrasonic channel. However, the surface layer preferably extends upto an end of the ultrasonic channel facing away from the ultrasonicconverter. In other words, it extends up to the surface of the holder inthe radiating direction or out of the entrance direction. In this mannerit is also possible to reduce the reflection of ultrasound impinging onthe holder immediately beside the ultrasonic channel.

In a plane transverse to the radiating direction it can likewise besufficient that only a sector of the surface limiting the ultrasonicchannel is formed by the surface layer. This can be desirable inparticular when the radiating or receiving characteristic is to differfor certain spatial directions. It is preferable, however, that thesurface layer encloses the portion of the ultrasonic channel at least inring-like fashion. This makes it possible to achieve an attenuation ofstanding waves or multiple reflections that is better and lessdirectional regarded in a plane transverse to the direction of radiatedor received ultrasound. Furthermore, manufacturing can be simplified.

In principle the holder can be formed of one piece, the surface layerbeing formed by a change in the composition of the material of theholder. For example, the holder can have a layer structure. According toa first variant, however, the holder can also comprise a base body witha receiving means and a sleeve-shaped portion disposed at least partlyin the receiving means, or a sleeve disposed at least partly in thereceiving means, said portion or sleeve comprising the surface layer andhaving the ultrasonic converter disposed at least partly therein. Saidportion or sleeve in which the portion of the ultrasonic channel extendscan be connected to the base body in the receiving means inforce-fitting and/or form-fitting and/or material-locking fashion, i.e.for example by friction or clamping and or by material-lockingconnection by means of a material for producing an adhesive connection.

The sleeve or sleeve-shaped portion of a suitable body can be obtainedfor example by processing a blank or by using a suitable mold uponproduction from liquid or pasty materials.

Alternatively, it is possible that the holder is formed by a base bodymade of the material of the surface layer, whereby material of thehigher impedance is disposed around the surface layer or the receivingarea in a direction transverse to the radiating direction or receivingdirection.

In principle the material used for the surface layer can be any materialhaving a suitable impedance. However, the surface layer is preferablyformed from a material whose average density is at least 10% smallerthan the average density of the material of the inside layer. Theaverage density is understood here to be the normal density in the caseof homogeneous materials, and in the case of inhomogeneous materials thedensity of areas that are much greater than the essential dimensions ofthe structural inhomogeneities, in the case of matrix composites forexample of the areas embedded in the matrix. This has the advantage thatfor influencing the impedance of a material its density can beinfluenced more easily than the sound velocity.

In principle the density of the surface layer can be chosen at will aslong as the impedance of the surface layer is smaller than that of theinside layer. It has been found that the density of the material of thesurface layer is preferably between 0.3 g/cm³ and 0.8 g/cm³. Anespecially effective reduction of standing waves or multiple reflectionresults when the density of the surface layer assumes a value between0.4 g/cm³ and 0.7 g/cm³.

The density of the inside layer is also in principle arbitrary, but ispreferably in the range between 0.8 g/cm³ and 1.8 g/cm³.

In principle the material used for the surface layer can be ahomogeneous material. However, the surface layer preferably comprises acomposite material with a weight proportion of the surface layer of morethan 80%. Particularly preferably, the surface layer comprises thecomposite material substantially, i.e. at more than 95 wt. %.

The composite material may in principle be any isotropic or anisotropiccomposite material. It is preferable to use a composite material whereinbodies or areas are embedded in a matrix that do not have thecomposition of the matrix. It is particularly preferable to use acomposite material with a matrix, and bodies distributed therein in adisordered order on which a scattering and/or absorption of ultrasoundat least of the predetermined frequency can be effected. This permitsthe occurrence of standing waves or the strength of multiple reflectionsto be reduced further.

It has been found that an especially good reduction of standing waves ormultiple reflections is surprisingly achieved when the material of thesurface layer comprises a syntactic foam and preferably consiststhereof. A syntactic foam is understood to be in particular a materialthat comprises a matrix material with hollow bodies, preferably hollowspheres, embedded therein. The syntactic foam is preferably transmissiveat least partly to the ultrasound of the predetermined frequency, sothat an at least small portion can pass through the surface layer ontothe inside layer. This kind of composite material can have the advantagethat a scattering of the ultrasound can partly take place on the hollowbodies, which impairs its propagation.

In particular for application in the area of the checking of valuedocuments, the syntactic foam can contain hollow bodies, in particularhollow spheres, with a diameter between 5 μm and 200 μm. In addition,the syntactic foam can also have hollow bodies of different diameter,but their proportion of the number of hollow bodies, based on the volumeof the foam, is preferably smaller than 20%. Particularly preferably,the range in which the diameters of the hollow bodies lie can be therange between 5 μm and 120 μm. The diameter of one of the hollow bodiesis understood here to be in particular the diameter of the smallestsphere enclosing the hollow body, i.e. in the case of hollow spheres inparticular their normal diameter. The hollow bodies can be distributedmonodispersely or uniformly with regard to their diameters in this rangeor, preferably, have a bimodal or multimodal distribution. The use of abimodal or multimodal distribution can offer the advantage that, uponthe preferred use of hollow spheres as hollow bodies, a substantiallyhigher packing density of the hollow spheres can be obtained, whichleads to reduced density and thus impedance and possibly to elevatedscattering.

The hollow spheres can fundamentally be manufactured from any material.Preferably, the syntactic foam contains hollow glass spheres, preferablyones whose diameter is between 5 μm and 200 μm. The use of hollow glassspheres has the advantage that they are readily available, compared toother materials.

As the matrix material of the syntactic foam there can be used inprinciple any material in which the hollow bodies are embeddable.Preferably, the syntactic foam contains as the matrix material apolymeric material, particularly preferably a resin. Depending on therequirement, there can be used here for example epoxy resins orpolyurethane resins.

The thickness of the surface layer can be chosen for example independence on the predetermined frequency and the impedance of theinside layer. In particular, the surface layer can have a thicknessgreater than one quarter of the wavelength in the surface layercorresponding to the predetermined frequency and smaller than 10 mm.

The inventive apparatus need not possess only one ultrasonic converter.Rather, it is preferable that the apparatus has at least one furtherultrasonic converter and wherein the holder possesses at least onefurther ultrasonic channel in which the further ultrasonic converter isdisposed at least partly, the holder having at least one furthernon-fibrous surface layer adjacent to at least one further portion ofthe further ultrasonic channel, whose impedance is smaller than that ofa further inside layer of the holder adjacent to the further surfacelayer.

The further surface layer can preferably be configured according to atleast one of the previously described embodiments.

The then at least two ultrasonic converters, the correspondingultrasonic channels, and the corresponding surface and inside layersneed not necessarily have the same properties and the same structure,but this is preferably the case.

In particular in this case, it is preferable that the surface layers areformed for at least two of the ultrasonic channels or ultrasonicconverters by portions of a single molded part. This embodiment offersnot one but several advantages. For example, a subsequent equipping of abase body for the holder with surface layers which are formed bysleeve-shaped portions of the molded body can be effected in only oneworking step.

For a simple production it can prove to be especially favorable if theholder has at least two layers, one of which is formed by the moldedpart in which the corresponding portions of the ultrasonic channels canbe formed.

Furthermore, upon use of syntactic foams based on polymeric matrixmaterials, such a molded part can often be produced especially easily,without any mechanical processing being necessary.

The apparatus can preferably be used for examining value documents withultrasound. The object is hence achieved according to the firstalternative further by an ultrasonic sensor for examining valuedocuments with at least one inventive apparatus.

In particular, it is preferable in the case of the ultrasonic sensor,for example for detecting double or multiple removals of value documentsby a singler or for checking value documents, that said sensor isprovided for examining value documents by means of ultrasound passingtherethrough, that said sensor has at least one further inventiveapparatus, and that the apparatuses are disposed relative to each othersuch that their holders form a, preferably slit-shaped, detection areathrough which a value document is transportable, and the ultrasonicconverters of the apparatuses form at least one ultrasound path when onethereof is operated as a transmitter and the other thereof as areceiver. Such ultrasonic sensors permit examinations in transmissionwhich are at the same time very robust and informative.

With such ultrasonic sensors it is advantageously possible to furtherreduce the formation of standing waves or the occurrence or effect ofmultiple reflections when, in the case of the ultrasonic sensor, theholder of the apparatus or at least of one of the apparatuses has on thesurface facing the detection area or the value document upon anexamination at least one non-fibrous surface layer whose impedance issmaller than that of an inside layer of the holder adjacent to thesurface layer. The surface layer adjacent to the detection area in whicha value document is located upon an examination or through which it istransported can be configured in particular according to one or more ofthe above-mentioned embodiments of the surface layer on the ultrasonicchannels. Further, it can advantageously be configured as a molded partwhich at the same time has the sleeve-shaped portions.

The object is also achieved according to a second alternative by anultrasonic sensor for checking a value document in a detection area ofthe ultrasonic sensor by means of ultrasound at least of a predeterminedfrequency with a first ultrasonic converter acting as a transmitter forthe ultrasound and a second ultrasonic converter acting as a receiverfor the ultrasound, the ultrasonic converters being so disposed thatthey form an ultrasound path extending through the detection area, andat least one of the ultrasonic converters having a surface portioninclined relative to the ultrasound path for radiating ultrasound and/orfor receiving ultrasound.

The ultrasonic sensor according to the second alternative of theinvention thus has the two ultrasonic converters one of which isconfigured for emitting ultrasound at least of the predeterminedfrequency and the other for receiving ultrasound at least of thepredetermined frequency. The ultrasonic converters can otherwise beconfigured identically or differently.

Due to the incline of the surface portion relative to the ultrasoundpath, in particular a connecting line between the oscillating surfacesof the ultrasonic converters emitting or receiving the ultrasound,ultrasound is likewise emitted or received inclined to the direction theultrasound path. Since it also holds for a reflection of ultrasound on avalue document that the angle of incidence is approximately the angle ofreflection, reflected portions of the ultrasound emitted by the surfaceportion are reflected out of the area of the ultrasound path, so thatstanding waves do not arise, or at least not to such an extent, orreflected pulses, in particular also multiply reflected pulses, do notimpinge on one of the ultrasonic converters of the ultrasound pathagain, or only with very low strength. Further, portions of theultrasound impinging on the ultrasonic converters are reflected up outof the immediate area of the ultrasound path by the inclined surfacearea.

Compared with a solution in which the ultrasound path is inclined to thesurface of the value document to reduce the effect of reflections, thereresults a reduced installation space of the ultrasonic sensor.

Although it can in principle be sufficient that the surface portion islevel, the surface portion of the at least one ultrasonic converter ispreferably configured rotationally symmetrically around the direction ofthe ultrasound path. It is then either possible for a sound fieldsimilar to a sound cone to be emitted, so that upon reflection on forexample a value document the sound portions more greatly inclined to thedirection of the ultrasound path are directed out of the ultrasound pathby reflection and can hence no longer influence the ultrasonicconverters, or it is possible for portions impinging on the ultrasonicconverters to be reflected away from the ultrasound path outwardly.

The surface portion can in particular be of conical or frustoconicalconfiguration. This embodiment has the advantage that said surfaceportion can be produced especially easily and at the same time a verygood effect results.

To also obtain a good focusing of the emitted ultrasound or anangular-dependent receiving characteristic that is not too wide, thecone opening angle of the surface portion is preferably between 160° and176°.

To obtain a good transmitting or receiving efficiency, the surfaceportion preferably has a maximum external diameter that is between thewavelength in air corresponding to the frequency and ten times thewavelength in air. Air can be assumed at normal conditions according tothe DIN standard or at a temperature of 20° C. at a pressure of 1 barand a relative air humidity of 75%.

An especially small installation space results when the ultrasonicsensor has at least one transport element for guiding the value documentin the detection area, and when the ultrasonic converters are sodisposed that the ultrasound path forms an angle smaller than 5° with anormal to the surface of the value document. The transport element caninvolve for example straps or rollers or similar transport elementswhich determine position of the value document relative to theultrasonic sensor.

As mentioned above, only one of the ultrasonic converters needs to havean inclined surface portion. However, an especially good function isobtained if the other ultrasonic converter is also configured accordingto an embodiment or a combination of at least two of the embodimentspreviously mentioned in connection with the second alternative.

With such ultrasonic sensors it is advantageously possible to reduce theformation of standing waves or the occurrence or effect of multiplereflections further by providing, in the area of the ultrasonic channelthrough which ultrasound is radiated or received by the ultrasonicconverter, an ultrasound-absorbing layer, for example made of a fibrousmaterial, in particular felt. However, there can also be providedalternatively or additionally in the case of the ultrasonic sensor atleast one element limiting the detection area and having a non-fibroussurface layer whose impedance is smaller than that of an inside layer ofthe element adjacent to the surface layer. The element can be configuredin particular as a holder for the particular ultrasonic sensor. Thesurface layer adjacent to the detection area can further be configuredaccording to one or more of the above-mentioned embodiments of thesurface layer in the ultrasonic channels. Thus the impedance of thematerial of the surface layer is preferably in the range between 20% and75% of the impedance of the material of the inside layer. In this casean especially good suppression of standing waves or multiple reflectionsis obtained.

Furthermore, said layer can advantageously be configured as a moldedpart which at the same time has the sleeve-shaped portions.

This embodiment has the advantage that the ultrasound impinging on thesurface limiting the detection area is likewise not reflected or onlyreflected in a strongly attenuated form, which greatly limits theformation of standing waves or the occurrence of strong multiplereflections.

The object is achieved according to a third alternative by an ultrasonicsensor for checking a value document in a detection area of theultrasonic sensor by means of ultrasound of a predetermined frequencyhaving an ultrasonic converter acting as a transmitter for theultrasound, an ultrasonic converter acting as a receiver for theultrasound, and at least one holder for the ultrasonic converters,wherein the at least one holder limits the detection area at leastpartly and at least one portion, limiting the detection area, of the atleast one holder has in the surroundings of the ultrasonic converterdisposed in or on the holder a non-fibrous surface layer whose impedanceis smaller than that of an inside layer of the holder adjacent to thesurface layer.

In particular, the ultrasonic sensor can have a holder in each case forthe ultrasonic converters which limit the detection area at leastpartly, whereby at least one portion, limiting the detection area, ofthe holders has in the surroundings of the ultrasonic converter disposedin or on the holder a non-fibrous surface layer whose impedance issmaller than that of an inside layer of the holder adjacent to thesurface layer.

For the properties and features of the surface layer, the embodimentsdescribed with respect to the surface layer in the first alternative arelikewise advantageously applicable.

Thus, it is in principle possible to use as the material for the surfacelayer any materials having a suitable impedance. Preferably, the surfacelayer is formed from a material whose average density is at least 10%smaller than the average density of the material of the inside layer.The average density is understood here to be, in the case of homogeneousmaterials, the normal density and, in the case of inhomogeneousmaterials, the density of areas that are much greater than the essentialdimensions of the structural inhomogeneities, in the case of matrixcomposites for example the areas embedded in the matrix. This has theadvantage that the density of a material can be influenced more easilythan the sound velocity.

In principle, the density of the surface layer can be chosen at will aslong as the impedance of the surface layer is smaller than that of theinside layer. It has been found that the density of the material of thesurface layer is preferably between 0.3 g/cm³ and 0.8 g/cm³. Anespecially effective reduction of standing waves or multiple reflectionresults when the density of the surface layer assumes a value between0.4 g/cm³ and 0.7 g/cm³.

The density of the inside layer is also in principle arbitrary, but ispreferably in the range between 0.8 g/cm³ and 1.8 g/cm³.

In principle it is possible to use as the material of the surface layera homogeneous material. However, the surface layer preferably comprisesa composite material with a weight proportion of the surface layer ofmore than 80%. Particularly preferably, the surface layer comprises thecomposite material substantially, i.e. at more than 95 wt. %.

The composite material may in principle be any isotropic or anisotropiccomposite material. It is preferable to use a composite material whereinbodies or areas are embedded in a matrix that do not have thecomposition of the matrix.

It has been found that an especially good reduction of standing waves ormultiple reflections is surprisingly achieved when the material of thesurface layer comprises a syntactic foam, preferably consists thereof. Asyntactic foam is understood here to be in particular a materialcomprising a matrix material with hollow bodies, preferably hollowspheres, embedded therein. The syntactic foam is preferably transmissiveat least partly to the ultrasound of the predetermined frequency, sothat an at least small portion can pass through the surface layer ontothe inside layer. This kind of composite material has the advantage thata scattering of the ultrasound can partly take place on the hollowbodies, which impairs its propagation.

In particular for the application in the area of the checking of valuedocuments, the syntactic foam can contain hollow bodies, in particularhollow spheres, with a diameter between 5 μm and 200 μm. The syntacticfoam can also contain hollow bodies of different diameter, but theirproportion of the number of hollow bodies, based on the volume of thefoam, is then preferably smaller than 20%. Particularly preferably, therange in which the diameters of the hollow bodies lie can be the rangebetween 5 μm and 120 μm. The diameter of one of the hollow bodies isunderstood here to be in particular the diameter of the smallest spherewhich encloses the hollow body, in the case of hollow spheres i.e. inparticular their normal diameter. The hollow bodies can be distributedmonodispersely with regard to their diameters in this range or have abimodal or multimodal distribution. The use of a bimodal or multimodaldistribution can offer the advantage that, upon the preferred use ofhollow spheres as hollow bodies, a substantially higher packing densityof the hollow spheres is obtained.

The hollow spheres can fundamentally be manufactured from any material.Preferably, the syntactic foam contains hollow glass spheres, preferablyones whose diameter is between 5 μm and 200 μm. The use of hollow glassspheres has the advantage that they are readily available, compared toother materials.

As the matrix material of the syntactic foam there can be used inprinciple any materials in which the hollow bodies are embeddable.However, the syntactic foam preferably contains as the matrix material apolymeric material, particularly preferably a resin. Depending on therequirement, there can be used here for example epoxy resins orpolyurethane resins.

The thickness of the surface layer can be chosen for example independence on the predetermined wavelength range and the impedance ofthe inside layer. In particular, the surface layer can have a thicknessgreater than one quarter of the wavelength in the surface layercorresponding to the predetermined frequency and smaller than 10 mm.

Further, the ultrasonic sensor can have at least one or a combination ofat least two of the features of the ultrasonic sensor according to thefirst alternative. In particular, there can, in the case of theultrasonic sensor, be formed at least in one of the holders at least oneultrasonic channel in which the particular ultrasonic converter isdisposed at least partly and/or through which ultrasound can pass fromor to the ultrasonic converter, the holder having at least onenon-fibrous surface layer adjacent to at least one portion of theultrasonic channel and whose impedance is smaller than that of an insidelayer neighboring to the surface layer of the holder. Preferably, thematerial of the surface layer adjacent to the ultrasonic channel and ofthe surface layer limiting the detection area are identicallyconfigured, although this need not necessarily be the case.

All stated alternatives offer the advantage that two measurements can beperformed at a relatively short time interval due to the strongattenuation or suppression of pulses reflected by the value document.Typically it is necessary to wait after the emission of an ultrasonicpulse intended for measurement until all echoes of the pulse, i.e.singly or multiply reflected parts of the pulse, have faded away farenough so as not to impair the detection of the transmitted measuringpulse for the next measurement. If fewer or weaker pulse echoes occur,the next measuring pulse can hence be emitted faster.

If the attenuation of a reflected pulse is very strong, the length ofthe ultrasound path can also be reduced, which likewise reduces the timerequired for measurement, which is determined by the pulse transit time,and the installation space for the particular ultrasonic sensor.

The object can further be achieved by an ultrasonic sensor for checkinga value document in a detection area of the ultrasonic sensor by meansof ultrasound of a predetermined frequency having an ultrasonicconverter acting as a transmitter for the ultrasound and an ultrasonicconverter acting as a receiver for the ultrasound, a holder portion ineach case for the transmitter and the receiver which have ultrasonicchannels flush with each other in each case in which the transmitter andreceiver are disposed in completely countersunk fashion so as to form anultrasound path extending through the detection area, and wherein theultrasonic converters are disposed so as to be inclined to an axis ofalignment of the ultrasonic channels.

Due to the inclined arrangement of the ultrasonic converters there isachieved a similar effect as with the ultrasonic sensor of the secondalternative, whereas it is possible here to use ultrasonic convertershaving level, non-inclined surfaces, for example standard ultrasonicconverters.

The above-described embodiments for the lining of the ultrasonic channelor the surface of a holder are advantageously applicable here too,because the laterally drifting echoes then greatly lose intensity uponeach reflection.

Particularly preferably, the surfaces of the ultrasonic channels and thesurface of the holder portions here too have a fibrous sound-absorbingor sound-insulating material or preferably a non-fibrous material asdescribed above.

The inventive sensors can be used especially advantageously invalue-document processing apparatuses. The subject of the invention ishence also a value-document processing apparatus for processing valuedocuments with an inventive apparatus and/an inventive ultrasonicsensor. Preferably, the value-document processing apparatus has atransport device for transporting value documents to the apparatus orthe ultrasonic sensor, in particular into or through the detection areaof the ultrasonic sensor, and for carrying off value documents from theultrasonic sensor, more precisely out of the detection area of theultrasonic sensor. A processing of value documents is understood here tobe in particular a check for authenticity of the value documents, fortheir condition, for example with regard to fitness for circulation, asort or also a count according to number and/or value. The ultrasonicsensor can be used here in particular also for checking for double ormultiple removals of value documents from a stack by a singler or forchecking the condition of a value document, in particular also of thepresence of a film, for example an adhesive film, on the value document.

The invention will hereinafter be explained more closely with referenceto the drawings. Therein are shown:

FIG. 1 a schematic representation of a bank-note processing apparatus,

FIG. 2 a schematic side view of an ultrasonic sensor of the bank-noteprocessing apparatus in FIG. 1,

FIG. 3 a schematic plan view of a holder of the ultrasonic sensor inFIG. 2 with ultrasonic converters held therein,

FIG. 4 a schematic view of a section through a portion of the holder inFIG. 3 along a plane orthogonal to a surface of the holder,

FIG. 5 a schematic plan view of a molded part of a holder of a secondultrasonic sensor,

FIG. 6 a schematic view of a section through a portion of the holder ofthe second ultrasonic sensor,

FIG. 7 a schematic view of a section through a portion of a holder of athird ultrasonic sensor along a plane orthogonal to a surface of theholder,

FIG. 8 a schematic plan view of a plate of the holder in FIG. 7,

FIG. 9 a schematic plan view of a holder of a fourth ultrasonic sensorwith ultrasonic converters held therein,

FIG. 10 a schematic view of a section through the fourth ultrasonicsensor along a plane orthogonal to a surface of a holder of theultrasonic sensor,

FIG. 11 a schematic view of a section through a portion of a holder of afifth ultrasonic sensor along a plane orthogonal to a surface of theholder,

FIG. 12 a schematic plan view of a plate of the holder in FIG. 11,

FIG. 13 a schematic view of a section through a portion of a holder of asixth ultrasonic sensor along a plane orthogonal to a surface of theholder, and

FIG. 14 a schematic view of a section through a seventh ultrasonicsensor along a plane orthogonal to a surface of a holder of theultrasonic sensor.

FIG. 1 shows an apparatus 10 for determining a condition of valuedocuments, in the example a bank-note processing apparatus which servesinter alia to determine the condition of value documents 12 in the formof bank notes. The apparatus 10 has an input pocket 14 for the input ofvalue documents 12 to be processed, a singler 16 which can access valuedocuments 12 stacked in the input pocket 14, a transport device 18 witha gate 20, and after the gate 20 an output pocket 26 and a shredder 28for destruction of value documents or bank notes. Along a transport path22 given by the transport device 18 there is disposed before the gate 20and after the singler 16 a sensor assembly 24 which is used fordetecting properties of singly supplied value documents 12 and formingsensor signals representing said properties. A control and evaluationdevice 30 is connected at least to the sensor assembly 24 and the gate20 via signal connections and is used for evaluating sensor signals ofthe sensor assembly 24 and controlling at least the gate 20 independence on the result of evaluation of the sensor signals, i.e. ofthe properties of the checked value documents 12 as detected by means ofthe sensor assembly.

The sensor assembly 24 comprises for this purpose at least one sensor;in this exemplary embodiment there are three sensors provided, namely, afirst sensor 32, in the example an optical sensor for detecting colorproperties which detects optical radiation reflected by the valuedocument, a second sensor 34, in the example likewise an optical sensorfor detecting special spectral security features of the value documentswhich detects optical radiation transmitted by the value document, and athird sensor 36, in the example an acoustic sensor, more precisely anultrasonic sensor which detects ultrasonic signals coming from, inparticular transmitted by, the value document.

Adhesive strips on the value documents 12 can be recognized for exampleby means of the sensor 36. For characterizing the condition of the banknotes the control and evaluation device 30 can for this purposedetermine for example the number of adhesive strips or the total lengthor total area of the adhesive strips from the sensor signals of thesensor 36.

While a value document 12 is transported past, the sensors 32, 34 and 36detect, according to their function, properties of scan areas on thevalue document determined by the position of the sensors relative to thevalue document, whereby the corresponding sensor signals are formed.Each of the sensors can have a different spatial resolution, i.e. thesize and distribution of the detected scan areas on the value documentcan vary in dependence on the particular sensor and the employedtransport speed. Each of the scan areas has associated therewith a placewhich represents the position of the scan areas for the particularsensor relative to each other and/or relative to the value document.

From the analog or digital sensor signals of the sensors 32, 34, 36 thecontrol and evaluation device 30 determines upon a sensor signalevaluation at least one property of at least one scan area and/or atleast one value-document property which are relevant for the checking ofthe bank notes with regard to their condition. Preferably, a pluralityof said properties are determined. Further, the authenticity of thevalue documents is checked by means of the signals of the sensor 34. Thevalue-document properties characterize the condition of the valuedocuments, in this example the condition of the bank notes with regardto fitness for use or fitness for circulation, i.e. their suitability tobe further employed as a means of payment. As correspondingvalue-document properties there are employed in this example inparticular the presence of soiling or stains and the presence of tears,adhesive strips, dog's-ears and/or holes, and/or the lack of parts ofthe value documents. Said value-document properties can be determined independence on sensor signals of only one of the sensors or of at leasttwo of the sensors.

The control and evaluation device 30 has for this purpose in particular,besides corresponding interfaces for the sensors, a processor 38 and amemory 40 connected to the processor 38 and storing at least onecomputer program with program code upon whose execution the processor 38controls the apparatus or evaluates the sensor signals, in particularfor determining an overall condition of a checked value document, anddrives the transport device 18 in accordance with the evaluation.

In particular, the control and evaluation device 30, more precisely theprocessor 38 therein, can, after determination of the value-documentproperties, check a criterion for the overall condition of the valuedocument which includes at least one of the value-document properties ordepends on at least one of the value-document properties. The criterioncan include in particular further reference data for defining apermissible condition of the value document which are predetermined andstored in the memory 40. The overall condition can be given for exampleby two categories, “still fit for circulation” or “fit for use”, and “tobe destroyed”. In dependence on the determined condition, the controland evaluation device 30, in particular the processor 38 therein,controls the transport device 18, more precisely the gate 20, such thatthe checked value document is transported, according to its determinedoverall condition, for deposit to the output pocket 26 (if still fit) orfor destruction to the shredder 28 (if to be destroyed).

For the processing of value documents 12, value documents 12 insertedinto the input pocket 14 as a stack or singly are singled by the singler16 and supplied singly to the transport device 18 which supplies thesingled value documents 12 to the sensor assembly 24. The latter detectsat least one property of the value documents 12, whereby sensor signalsare formed which represent the property of the value document. Thecontrol and evaluation device 30 detects the sensor signals, determinesin dependence thereon a condition of the particular value document, anddrives the gate 20 in dependence on the result such that, for example,still employable value documents are supplied to the output pocket 26and value documents to be destroyed to the shredder 28 for destruction.

For determination of the overall condition of the bank notes, thecontrol and evaluation device 30 employs the above-mentioned criterionwhich can include at least one of the properties. The individual valuescan preferably for example be linked in a criterion, for example bymeans of a linear combination. The control and evaluation device 30then, for determining the overall condition of the bank notes, comparesthe linear combination of the properties characterizing the condition ofthe bank notes to a predetermined value and decides for example whetherthe condition of the bank notes is good or bad, i.e. whether or not theyare fit for circulation. This permits a bank note that already hasconsiderable soiling but in isolation not enough to result in thecondition of the bank note being determined as bad, to be determined asbad when the bank note additionally has e.g. only a few stains and/ortears, etc.

The ultrasonic sensor 36, an ultrasonic sensor according to a firstpreferred embodiment of the invention, in particular the firstalternative, is partly shown more precisely in FIGS. 2 to 5.

The ultrasonic sensor 36 is provided for examining value documents 12 bymeans of ultrasound passing therethrough and possesses for this purposetwo apparatuses 42 and 42′ for emitting or receiving ultrasound in apredetermined frequency range which are identically configured accordingto a first preferred embodiment of the invention and except for amirroring on a plane.

The apparatuses 42 and 42′ for emitting or receiving ultrasound in thepredetermined frequency range have in each case at least one, in thisexample eighteen, ultrasonic converters 44 for converting electricalsignals into ultrasound or converting ultrasound into electricalsignals, and a holder 46 or 46′ in each case in which a number ofultrasonic channels 48 corresponding to the number of ultrasonicconverters is formed, in the example by portions of through holes 50, inwhich one of the ultrasonic converters 44 is in each case disposed atleast partly and through which ultrasound can pass from or to theparticular ultrasonic converter 44.

The ultrasonic sensor 36 further possesses a driving andsignal-processing device 52 which is connected via only schematicallyshown electrical connections 54 to the ultrasonic converters 44 and, notshown in FIG. 2, to the control and evaluation device 30 for theexchange of signals and supplying power,

The holders 46 and 46′ are configured in a plate-like manner in theirbasic shape and disposed with their plate surfaces 56 parallel to eachother, so that the latter are aligned in a direction orthogonal to theplate surfaces 56. The transport path 22 extends through a gap formedbetween the holders 46 and 46′ which at the same time constitutes adetection area 58, so that value documents 12 transported through thedetection area 58 can be examined by means of ultrasound intransmission. In other words, the apparatuses 42 and 42′ are disposedrelative to each other such that the ultrasonic converters of theapparatuses form ultrasound paths 43 when, of two ultrasonic converters44 opposing each other in a direction orthogonal to the plate surfaces56, one is operated as a transmitter and the other as a receiver. Asillustrated in FIG. 2, the ultrasonic converters 44 are thus so disposedthat the ultrasound paths 43 formed between transmitters and receiverextend in good approximation orthogonally to the value document 12 or tothe plate surfaces 56.

More precisely, the ultrasonic converters 44 in the holder 46 areemployed or driven as ultrasound transmitters, while the ultrasonicconverters 44 in the holder 46′ serve as receivers for ultrasound whichhas passed through a value document 12 or is radiated thereby. Theultrasonic converters serving as receivers emit corresponding detectionsignals upon detection of ultrasound.

The ultrasonic converters 44 are so configured in this exemplaryembodiment as to be suitable for converting ultrasound with apredetermined frequency of about 400 kHz. To also permit convert ofpulses with a duration smaller than 100 μs, they are preferablyconfigured for converting ultrasound in a frequency range with a widthof about 70 kHz, in the middle of which the predetermined frequencylies.

Each of the ultrasonic converters 44, which are cylindrical in theexample, has a holding groove 60 along its circumference. Further, ithas on its front side facing away from the detection area 58 twoelectrical contacts, in the example the connection wires 54, for director indirect electrical connection of the ultrasonic converter 44 to thedriving and signal-processing device 52.

The holder 46, which, as described above, is configured like the holder46′ except for the mirroring, is shown more precisely in FIGS. 3 and 4.

The holder 46 has, firstly, a plate-shaped base body 62 in which theidentically configured receiving means 63, in the example through holes,extending orthogonally to the plate surfaces 56 of the holder 46 or basebody 62 are formed in three mutually offset rows at equal intervals ineach case.

Secondly, the holder 46 has on a portion of the inner side of eachthrough holes 50 or in a portion of the receiving means 63 sleeveportions or hollow cylindrical portions 64 which are formed of adifferent material from the base body 62. The internal spaces of thesleeve portions 64 form ultrasonic channels 48 in which ultrasoundemitted or to be received by the particular ultrasonic converter can bepropagated.

The base body 62 is formed in this exemplary embodiment from a glassfiber-reinforced polymer, a synthetic resin, and has a thickness ofabout 8 mm. The employed glass fiber-reinforced synthetic resin has adensity of about 1.3 g/cm³ and for the predetermined frequency animpedance of about 3·10⁶ kg/(m² s).

The sleeve portions 64, in contrast, are manufactured from a compositematerial, and thus in particular a composite material with a volumeproportion of the sleeve portion 64 of more than 80%, in this exemplaryembodiment a syntactic foam at least partly transmissive to ultrasoundof the predetermined frequency, and form a non-fibrous surface layer 66whose impedance is smaller than that of the base body 62 and thus of aninside layer 68 of the base body 62, and thus of the holder 46, that isneighboring to the surface layer 66, in particular adjacent thereto.

A cross section through the holder 46 in the area of one of the throughholes 50 is shown schematically in FIG. 5. The other correspondingportions of the holder 46 are configured identically.

Each through hole 50 has substantially two portions 70 and 72 withdifferent diameters, of which the portion 70, which opens on the surface56 facing the detection area, has the greater diameter. Thus there isformed an annularly extending shoulder 76 in a middle portion on aninternal surface or wall 74, forming the through hole 50, of the holder46. Said portion 70 also comprises the sleeve portion 64.

The ultrasonic converter 44 is held via a holding element, in theexample a circular ring 78, consisting of elastic, preferablysound-absorbing material which is inserted with prestressing into theholding groove 60, on the shoulder 76 so as to be aligned in a directionpredetermined by the position of the shoulder 76 and the position of theholding groove 60 relative to the holder 46, in the example in FIGS. 2to 5 in a direction at least approximately orthogonal to the platesurfaces 56. This results in a corresponding alignment of the receivingor transmitting characteristic (sensitivity) relative to the holder 46;the maximum and thus the radiating direction A lies approximately in adirection at least approximately orthogonal to the plate surfaces 56.Thus, with the shown arrangement of the holders 46 and 46′, thetransmitting and the receiving characteristics of the ultrasonicconverters of a particular ultrasound path are aligned exactly with eachother.

The material of the sleeve portions 64 and thus of the surface layers 66formed thereby has a density between 0.3 g/cm³ and 0.8 g/cm³, preferablybetween 0.4 g/cm³ and 0.7 g/cm³.

The composite material of the sleeve portions 64 comprises a matrix withembedded bodies. In this exemplary embodiment it is, more precisely, asyntactic foam which contains in a matrix material hollow bodies, herehollow spheres, with a diameter between 5 μm and 200 μm, preferablybetween 5 μm and 120 μm, whereby the matrix can also contain hollowspheres of different size. The diameter distribution of the hollowspheres is so chosen that a high packing density results, although inother exemplary embodiments a monodisperse distribution can also beemployed. The diameter distribution can in particular be bimodal.

In the present example, the syntactic foam contains hollow glass spheresas hollow bodies, whose diameter is preferably between 10 μm and 200 μm.

As the matrix material the syntactic foam here comprises a polymericmaterial, in particular a resin. In this exemplary embodiment, thepolymeric material is an epoxy resin.

In this exemplary embodiment, the syntactic foam and thus the surfacelayer 66 possesses an average density of 0.5 g/cm³, which is thus atleast 10% lower than that of the inside layer 68, and a sound velocityfor ultrasound of the predetermined frequency of about 2500 m/s, i.e. animpedance of about 1.3·10⁶ kg/(m² s), which is thus between 20% and 75%of the impedance of the base body 62 and thus of the inside layer 68, inthe example about 43%.

The surface layer 66 formed by the particular sleeve portion 64 henceencloses the ultrasonic channel in ring-like fashion, so that a uniformchange in acoustic behavior is obtained by the surface layer. The insidelayer 68 of higher impedance in turn encloses the surface layer 66.

The thickness of the surface layer 66, i.e. the wall thickness of thesleeve portion 64, is greater than one quarter of the wavelength in thesurface layer 66 corresponding to the predetermined frequency andsmaller than 10 mm, in this example about 1.5 mm.

Further, the sleeve portion 64 and thus the surface layer 66 extends inthis exemplary embodiment in a direction orthogonal to the surface 56 orparallel to the ultrasound channel or the radiating direction A from thesurface 56 up to the holding device for the ultrasonic converter 44,here the shoulder 76.

In this exemplary embodiment, the holding groove 60 and the shoulder 76are so configured in connection with the thickness of the holder 46 thatthe ultrasonic converter 44 is countersunk at least partly, in theexample completely, in the ultrasonic channel 48.

The sleeve portion 64 is inserted substantially in form-fitting fashioninto the receiving means 63 in the base body 62 and connected inmaterial-locking fashion thereto. The material locking is obtained inthe present example by an epoxy resin adhesive which in the cured statehas an impedance that corresponds to that of the inside layer 68 and isin particular not significantly greater than that of the inside layer68.

This configuration of the holders 46 and 46′ makes it possible to obtaina strong reduction of the amplitude of singly or multiply reflectedpulses or of the formation of standing waves in continuous operation.

Further, the ultrasonic converters 44 are well protected by thecountersinking in the through holes 50 against mechanical action in adirection parallel to the plate surfaces.

As an optional feature, for protection of the contacting of theconnecting lines 54 on the ultrasonic converter 44 from mechanical andchemical influences and for further mechanical support of the ultrasonicconverter 44 in the through hole 50, the area of the through hole 50from the ultrasonic converter 44 up to the plate surface 56 is cast witha polymeric material 80 which is electrically insulating and elastic atleast to the extent that it can compensate difference in the dimensionalchanges of ultrasonic converters and holes caused by temperaturevariations. Said material is omitted in FIG. 3 for clarity's sake.

The holder 46, finally, has mounting holes 82 for fastening in thevalue-document processing apparatus.

A second exemplary embodiment, which at the same time constitutes apreferred embodiment according to the third alternative of theinvention, differs from the first exemplary embodiment only in that thesurface layers 66 are provided in a different way and the holders aremodified accordingly. All the other components and features are the sameas in the first exemplary embodiment, so that the same reference signsare employed therefor and the description thereof and of their functionalso applies here accordingly.

As shown schematically in FIGS. 5 and 6, a molded part 84 is nowprovided which is manufactured from the same material as the surfacelayers 66 in the first exemplary embodiment.

The molded part 84 has a plate portion 86 with through holes 88 which,when the plate portion 86 is placed accordingly on the base body 62, areflush with the through holes 50 therein. Around the through holes 88 orflush therewith there are disposed cylindrical sleeve portions 90 whosedimensions correspond to those of the sleeve portions 64 in the firstexemplary embodiment. However, said sleeve portions 90 are connectedintegrally to the plate portion 86, so that the molded part 84 isone-pieced. In this exemplary embodiment the molded part 84 is producedby injection molding.

Said molded part 84 is inserted with the sleeve portions 90, whoseinternal spaces again form ultrasonic channels, into the receiving means63 in the base body 62 and connected thereto to form a holder 46″, whichcan be obtained for example by bonding by means of a suitable adhesive,for example, epoxy resin. In this manner there firstly result surfacelayers 66′, corresponding to the surface layers 66 of the firstexemplary embodiment, in the walls of the ultrasonic channels 48 for theultrasonic converters 44. Secondly, the base body 62 is now covered onthe side facing the detection area 58 by the plate portion 86, so thatthe holder 46″ has on the surface 56 facing the value document 12 uponan examination a non-fibrous surface layer 92 formed by the plateportion 86 and whose impedance is smaller than that of an inside layer94 of the holder 46″ adjacent to the surface layer, which is formed by acorresponding portion of the base body 62. The impedance differences arethe same as in the first exemplary embodiment.

The other holder, not shown, is constructed identically to the holder46″ except for a mirroring, as in the first exemplary embodiment.However, this need not necessarily be the case in other exemplaryembodiments.

This configuration of the holders also makes it possible to avoidreflections on the surfaces of the holders and thus in the channelformed thereby, which can additionally also reduce crosstalk betweenneighboring ultrasound paths.

Further, the production of the sleeve portions and their assembly isfacilitated, which can be a considerable advantage in particular withultrasonic sensors having a great number of ultrasound paths.

In other exemplary embodiments it is also possible to employ syntacticfoams with a different, in particular polymeric, matrix material. Forexample, polyurethane can also be used.

In yet other exemplary embodiments, the direction of the ultrasonicchannels 48 or the principal radiating and receiving direction A of theultrasonic converters 44 and thus of the ultrasound path can be inclinedto the surface 56 and thus to a value document 12 to be examined.

Further exemplary embodiments can differ from the previously describedexemplary embodiments in that instead of the through holes 50 there areprovided blind holes in which the ultrasonic converters 44 are disposedat least partly.

In other exemplary embodiments, the through holes can also have holdingstructures on their inner side, for example projections in the form oflugs, which immediately hold the ultrasonic converters 44.

Yet other exemplary embodiments can differ from the previously describedexemplary embodiments in that the sleeve portions are so configured thatthey serve as receiving means or holding devices for the ultrasonicconverters. For this purpose, the sleeve portions can be equipped ontheir inner side, like the receiving means 63, with a holding structure,for example projections or a circumferential shoulder.

Furthermore, further exemplary embodiments can differ from the describedexemplary embodiments in that the ultrasonic converters protrude partlyout of the ultrasonic channels 48.

Further, it is possible that the base body 62 in the exemplaryembodiments has at least two interconnected, in particularglued-together or mutually adherent, layers.

An ultrasonic sensor according to a third preferred embodiment of theinvention, more precisely a preferred embodiment of the secondalternative according to the invention, is illustrated schematically inFIGS. 7 and 8.

Said ultrasonic sensor differs from the ultrasonic sensor of the secondexemplary embodiment only by the configuration of the holder 46′″. Allthe other elements of the exemplary embodiment fail to differ from thesecond exemplary embodiment, so that the same reference signs areemployed for the same elements and the explanations thereof also holdaccordingly here. The employed materials are the same as in the secondexemplary embodiment.

As illustrated in FIGS. 7 and 8, the holder 46′″ differs from the holder46″ firstly in that instead of the molded part 84 there is now employeda plate-shaped molded part 84′ which no longer has the sleeve-likeportions 90 and is hence given substantially by a plate in which thereare formed, besides the holes 82, through holes 88 for forming portionsof the ultrasonic channels 48. The thickness of the molded part 84 isnot shown true to scale in the figures. It can be about 5 mm in theexample.

Since the sleeve portions 90 are now absent, the base body 62′ differsfrom the base body 62 in that the through holes 50 now have in the areaof the portion 70 an inside diameter corresponding to that of theultrasonic channels and thus of the through holes 88.

As illustrated in FIG. 7, the inside layer 94′ of higher impedance nowextends in the base body 62′ parallel to the molded part 84′ and thus tothe non-fibrous surface layer 66″ of lower impedance and approximatelyorthogonal to direction A of the ultrasonic channels 48 which are nowadjacent to each other in direction A.

An ultrasonic sensor according to a fourth preferred embodiment of theinvention, more precisely a preferred embodiment of the secondalternative according to the invention, is illustrated schematically inFIGS. 9 and 10.

Said ultrasonic sensor 96 replaces the ultrasonic sensor 36 in the firstexemplary embodiment. The apparatus 10 for determining a condition ofvalue documents is otherwise unchanged.

The ultrasonic sensor 96 differs from the ultrasonic sensor 36 firstlyby the use of holders 98 instead of the holders 46, 46′ and secondlyultrasonic converters 100 modified compared with the ultrasonicconverters 44. Otherwise the ultrasonic sensor is unchanged and thusconstitutes an ultrasonic sensor for checking a value document 12 in adetection area 58 of the ultrasonic sensor by means of ultrasound atleast of a predetermined frequency having an ultrasonic converter actingas a transmitter for the ultrasound and an ultrasonic converter actingas a receiver for the ultrasound, wherein the ultrasonic converters areso disposed as to form an ultrasound path 102 extending through thedetection area 58. The statements on the unchanged parts and features ofthe ultrasonic sensor of the first exemplary embodiment apply hereaccordingly, and the same reference signs are employed for the sameparts.

The holders 98 are changed compared with the holders 46 and 46′ only tothe effect that, while having the same form, they are manufacturedcompletely from the same material, which may in particular be thematerial of the base body 62 in the first exemplary embodiment (cf.FIGS. 9 and 10).

The ultrasonic converters 100 are changed compared with thecorresponding ultrasonic converters of the first exemplary embodiment tothe effect that these have 102 inclined surface portions 104 for theradiation of ultrasound and/or for the receiving of ultrasound onerelatively to the ultrasound path. This incline results firstly in theradiated ultrasound being propagated rather conically, so that portionsreflected on a checked value document 12 are directed out of the area ofthe ultrasound path. Secondly, on the receiver side, transmittedultrasound that is not converted by the particular ultrasonic converteris at least partly conically reflected, so that portions of thereflected ultrasound are directed out of the area of the ultrasound path102 here too. The portions directed out of the area of the ultrasoundpath can no longer form standing waves or lead to pulse echoes.

Said surface portions 104 adjacent to air as the transmission medium forthe ultrasound are configured to be cone-shaped or conical around thedirection of the ultrasound path 102 or the radiating direction A(averaged over the beam cross section) as the axis, so that they areconfigured to be rotationally symmetric around the direction of theultrasound path 102.

The cone opening angle α here is preferably between 160° and 176°, inthe present example about 170°. This choice achieves a good compromisebetween an alignment of the radiated ultrasound, on the one hand, and areduction of the occurrence of standing waves or of the strength ofspurious pulse reflections while the amplitude of the transmitted pulsesis at the same time sufficient for the checking purposes.

The surface portion 104, i.e. the conical portion, has a maximumexternal diameter, in a plane orthogonal to the axis of symmetry withregard to which the surface portion is rotationally symmetric, which isbetween the wavelength in air corresponding to the predeterminedfrequency and ten times the wavelength. In particular, the air can havea pressure of 1 bar, a temperature of 20° and a relative humidity of75%. In the example the maximum external diameter is about 6 mm.

As shown in FIG. 10, the ultrasonic sensor has at least one transportelement for guiding the value document 12 in the detection area 58,which in the present example is given by straps 106 between which thevalue document 12 is held for transport. The ultrasonic converters 100are disposed that the ultrasound path 102 forms an angle smaller than 5°with a normal to the surface of the value document 12.

A further exemplary embodiment differs from the fourth exemplaryembodiment in that the holders 98 are now replaced by the holders 46,46′ of the first exemplary embodiment or the second exemplaryembodiment.

A fifth preferred embodiment of the invention in FIG. 11 and FIG. 12differs from the fourth exemplary embodiment in that the holders 98 arereplaced by holders 108. All the other parts and features of the thirdexemplary embodiment are unchanged, so that the same reference signs areemployed therefor and the remarks thereon also apply here.

The holders 108 differ from the holders 88 only in that on a base body110 corresponding to the holder 98 there is provided a perforated plate112 firmly connected, for example bonded, to the base body 110 as thesurface layer facing the detection area. The surface layer isconstructed from a non-fibrous material whose impedance is smaller thanthat of the material of an inside layer 114 of the holder 108 adjacentto the surface layer. Said holder 108 forms an element limiting thedetection area and absorbs at least a portion of the ultrasoundimpinging thereon, which additionally lowers the influence of spuriousreflections.

The material of the plate 112 and thus of the surface layer formedthereby can have in particular a density between 0.3 g/cm³ and 0.8g/cm³, preferably between 0.4 g/cm³ and 0.7 g/cm³.

In this exemplary embodiment, the material is a composite material whichcomprises a matrix with embedded bodies. More precisely, it is asyntactic foam which contains in a matrix material hollow bodies, herehollow spheres, with a diameter between 5 μm and 200 μm, preferablybetween 5 μm and 120 μm, whereby the matrix can also contain hollowspheres of different size. The diameter distribution of the hollowspheres is so chosen that a high packing density results, although inother exemplary embodiments a monodisperse distribution can also beemployed. The diameter distribution can in particular be bimodal.

In the present example, the syntactic foam contains hollow glass spheresas hollow bodies, whose diameter is preferably between 10 μm and 200 μm.

As the matrix material the syntactic foam here comprises a polymericmaterial, in particular a resin. In this exemplary embodiment, thepolymeric material is an epoxy resin.

In this exemplary embodiment, the syntactic foam and thus the surfacelayer possesses an average density of 0.5 g/cm³, which is thus at least10% lower than that of the inside layer 68, and a sound velocity forultrasound of the predetermined frequency of about 2500 m/s, i.e. animpedance of about 1.3·10⁶ kg/(m² s), which is thus between 20% and 75%of the impedance of the base body 62 and thus of the inside layer 114,in the example about 43%.

The thickness of the plate 112, i.e. of the surface layer, is greaterthan one quarter of the wavelength in the surface layer 112corresponding to the predetermined frequency and smaller than 10 mm, inthis example more precisely 2 mm².

This exemplary embodiment hence embodies in particular also a preferredembodiment of the third alternative of the invention, namely, ultrasonicsensor for checking a value document in a detection area of theultrasonic sensor by means of ultrasound of a predetermined frequencyhaving an ultrasonic converter acting as a transmitter for theultrasound and an ultrasonic converter acting as a receiver for theultrasound, and a holder in each case for the ultrasonic converters,wherein the holders limit the detection area, and at least one portion,limiting the detection area, of at least one of the holders has, in thesurroundings of the ultrasonic converter disposed in or on the holder, anon-fibrous surface layer whose impedance is smaller than that of aninside layer of the holder adjacent to the surface layer.

In yet another exemplary embodiment, an ultrasonic converter of thefirst exemplary embodiment can be employed as the ultrasonic converterfor receiving ultrasound.

A further exemplary embodiment differs from the fifth exemplaryembodiment in that instead of the ultrasonic converters 100 there areused the ultrasonic converters 44 of the first exemplary embodiment.

A further variant of the fourth exemplary embodiment is shown in FIG.11. It differs from the fourth exemplary embodiment only by the form ofthe surface portion 116, which is now also inclined to the direction ofthe ultrasound path and configured to be rotationally symmetrictherearound. However, the surface portion 116 is lowered conicallytoward its center or against the radiating direction A of theultrasound.

In a seventh exemplary embodiment in FIG. 14, an ultrasonic sensor hasfor checking a value document in a detection area of the ultrasonicsensor by means of ultrasound of a predetermined frequency having anultrasonic converter 118 acting as a transmitter for the ultrasound andan ultrasonic converter 118 acting as a receiver for the ultrasound, aholder portion 120 and 122 in each case for the transmitter and thereceiver which have ultrasonic channels flush with each other in eachcase in which the transmitter and receiver are so disposed in completelycountersunk fashion as to form an ultrasound path extending through thedetection area. The ultrasonic converters 118 are disposed so as to beinclined to an axis of alignment of the ultrasonic channels. Theinclined arrangement of the ultrasonic converters relative to theultrasound path and the ultrasonic channel forms an ultrasound path butpossible echoes drift out laterally.

Particularly preferably, the surfaces of the ultrasonic channels and thesurfaces of the holder portions 120 and 122 here too have a layerconsisting of a fibrous sound-absorbing or sound-insulating material,for example a felt material, or preferably a non-fibrous material 130,as described above.

1. An apparatus for emitting and/or receiving ultrasound at least of apredetermined frequency, comprising: at least one ultrasonic converteradapted to convert ultrasound at least of the predetermined frequencyinto electrical signals and/or converting electrical signals intoultrasound at least of the predetermined frequency, and a holder havingat least one ultrasonic channel formed therein in which the ultra-sonicconverter is disposed at least partly and/or through which ultrasoundcan pass from or to the ultrasonic converter, wherein the holder has atleast one non-fibrous surface layer adjacent to at least one portion ofthe ultrasonic channel and whose impedance is smaller than that of aninside layer of the holder neighboring to the surface layer.
 2. Theapparatus according to claim 1, wherein the surface layer extends up toan end of the ultrasonic channel facing away from the ultrasonicconverter.
 3. The apparatus according to claim 1, wherein the surfacelayer encloses at least in ring-like fashion the portion of theultrasonic channel.
 4. The apparatus according to claim 1, wherein theholder comprises a base body having a receiving device and asleeve-shaped portion disposed at least partly in the receiving deviceor a sleeve disposed at least partly in the receiving device, saidportion or sleeve comprising the surface layer and having the ultrasonicconverter disposed at least partly therein.
 5. The apparatus accordingto claim 1, wherein the surface layer is formed from a material whoseaverage density is at least 10% smaller than the average density of thematerial of the inside layer.
 6. The apparatus according to claim 1,wherein the density of the material of the surface layer is between 0.3g/cm³ and 0.8 g/cm³.
 7. The apparatus according to claim 1, wherein thesurface layer comprises a composite material with a weight proportion ofthe layer of more than 80%.
 8. The apparatus according to claim 1,wherein the material of the surface layer comprises a syntactic foam. 9.The apparatus according to claim 8, wherein the syntactic foam containshollow bodies with a diameter between 5 μm and 200 μm.
 10. The apparatusaccording to claim 8, wherein the syntactic foam contains hollow glassspheres.
 11. The apparatus according to claim 8, wherein the syntacticfoam contains as the matrix material a polymeric material.
 12. Theapparatus according to claim 1, wherein the surface layer has athickness greater than one quarter of the wavelength in the surfacelayer corresponding to the predetermined frequency and smaller than 10mm.
 13. The apparatus according to claim 1, including at least onefurther ultrasonic converter, and wherein the holder includes at leastone further ultrasonic channel in which the further ultrasonic converteris disposed at least partly, the holder further including at least onefurther non-fibrous surface layer adjacent to at least one furtherportion of the further ultrasonic channel, whose impedance is smallerthan that of a further inside layer of the holder adjacent to thefurther surface layer.
 14. The apparatus according to claim 13, whereinthe further surface layer is configured according to at least claim 2.15. The apparatus according to claim 13, wherein the surface layers forat least two of the ultrasonic channels or ultrasonic converters areformed by portions of a single molded part.
 16. The apparatus accordingto claim 15, wherein the holder comprises at least two layers, one ofwhich is formed by the molded part in which the corresponding portionsof the ultrasonic channels are formed.
 17. An ultrasonic sensor forexamining value documents having at least one apparatus according toclaim
 1. 18. The ultrasonic sensor according to claim 17, adapted toexamine value documents by means of ultrasound passing therethrough,comprising at least one further apparatus according to claim 1, andwherein the apparatuses are disposed relative to each other such thattheir holders form a detection area through which a value document istransportable, and the ultrasonic converters of the apparatuses form atleast one ultrasound path when one thereof is operated as a transmitterand the other thereof as a receiver.
 19. The ultrasonic sensor accordingto claim 17, wherein the holder of the apparatus or of at least one ofthe apparatuses has on the surface facing the value document upon anexamination at least one non-fibrous surface layer whose impedance issmaller than that of an inside layer of the holder adjacent to thesurface layer.
 20. An ultrasonic sensor for checking a value document ina detection area of the ultrasonic sensor by means of ultrasound of apredetermined frequency, comprising an ultrasonic converter acting as atransmitter for the ultrasound and an ultrasonic converter acting as areceiver for the ultrasound, wherein the ultrasonic converters are sodisposed as to form an ultrasound path extending through the detectionarea, and wherein at least one of the ultrasonic converters has asurface portion inclined relative to the ultrasound path for radiatingultrasound and/or receiving ultra-sound.
 21. The ultrasonic sensoraccording to claim 20, wherein the surface portion of the at least oneultrasonic converter is configured to be rotationally symmetric aroundthe direction of the ultrasound path.
 22. The ultrasonic sensoraccording to claim 21, wherein the surface portion is of conical orfrustoconical configuration.
 23. The ultrasonic sensor according toclaim 22, wherein the cone opening angle of the surface portion isbetween 160° and 176°.
 24. The ultrasonic sensor according to claim 20,wherein the surface portion has a maximum external diameter which isbetween the wavelength in air corresponding to the frequency and tentimes the wavelength.
 25. The ultrasonic sensor according to claim 20,comprising at least one transport element for guiding the value documentin the detection area, and wherein the ultrasonic converters are sodisposed that the ultrasound path forms an angle smaller than 5° withrespect to a normal to the surface of the value document.
 26. Theultrasonic sensor according to claim 20, wherein the other ultrasonicconverter is also configured according to claim
 1. 27. The ultrasonicsensor according to claim 20, including at least one element limitingthe detection area which has a non-fibrous surface layer whose impedanceis smaller than that of an inside layer of the element adjacent to thesurface layer.
 28. An ultrasonic sensor for checking a value document ina detection area of the ultrasonic sensor by means of ultrasound atleast of a predetermined frequency, comprising: an ultrasonic converteracting as a transmitter for the ultrasound and an ultrasonic converteracting as a receiver for the ultrasound, and at least one holder for theultrasonic converters, wherein the at least one holder limits thedetection area at least partly, and at least one portion, limiting thedetection area, of the at least one holder has in the surroundings ofthe ultrasonic converter disposed in or on the holder a non-fibroussurface layer whose impedance is smaller than that of an inside layer ofthe holder adjacent to the surface layer.
 29. The ultrasonic sensoraccording to claim 27, wherein the surface layer is formed from amaterial whose average density is at least 10% smaller than the averagedensity of the material of the inside layer.
 30. The ultrasonic sensoraccording to claim 27, wherein the density of the material of thesurface layer is between 0.3 g/cm³ and 0.8 g/cm³.
 31. The ultrasonicsensor according to claim 27, wherein the surface layer comprises acomposite material with a volume proportion of the layer of more than80%.
 32. The ultrasonic sensor according to claim 27, wherein thematerial of the surface layer comprises a syntactic foam.
 33. Theultrasonic sensor according to claim 32, wherein the syntactic foamcontains hollow spheres with a diameter between 5 μm and 200 μm.
 34. Theultrasonic sensor according to claim 32, wherein the syntactic foamcontains hollow glass spheres.
 35. The ultrasonic sensor according toclaim 32, wherein the syntactic foam contains a resin as the matrixmaterial.
 36. The ultrasonic sensor according to claim 27, wherein thesurface layer has a thickness greater than one quarter of the wavelengthin the surface layer corresponding to the predetermined frequency andsmaller than 10 mm.
 37. The ultrasonic sensor according to claim 27,wherein, at least in one of the holders, at least one ultrasonic channelis provided in which the particular ultrasonic converter is disposed atleast partly and/or through which ultrasound can pass from or to theultrasonic converter, the holder having at least one non-fibrous surfacelayer adjacent to at least one portion of the ultrasonic channel, whoseimpedance is smaller than that of an inside layer of the holderneighboring to the surface layer.
 38. The ultrasonic sensor according toclaim 17, including a holder portion in each case for the transmitterand the receiver which have ultra-sonic channels flush with each otherin each case in which the transmitter and receiver are so disposed incompletely countersunk fashion as to form an ultra-sound path extendingthrough the detection area, and wherein the ultrasonic converters aredisposed so as to be inclined with respect to an axis of alignment ofthe ultrasonic channels.
 39. An ultrasonic sensor for checking a valuedocument in a detection area of the ultrasonic sensor by means ofultrasound of a predetermined frequency, comprising: an ultrasonicconverter acting as a transmitter for the ultrasound and an ultrasonicconverter acting as a receiver for the ultrasound; a holder portion ineach case for the transmitter and the receiver which have ultrasonicchannels flush with each other in each case in which the transmitter andreceiver are so disposed in completely countersunk fashion as to form anultra-sound path extending through the detection area, and wherein theultrasonic converters are disposed so as to be inclined relative to anaxis of alignment of the ultrasonic channels.
 40. A value-documentprocessing apparatus for processing value documents, comprising anapparatus according to claim 1 and/or an ultrasonic sensor according toclaim 17.